Color displays employing liquid crystal display (LCD) techniques have been the subject of intense research and development efforts in recent years. Such displays have low power requirements, making them particularly attractive for portable applications with limited power capacity, such as battery powered laptop-type computers. Further, LCD displays can be operated to selectively filter primary colors from light projected through the display's pixels to form a colored image. Such color LCD displays are sometimes employed in cooperation with overhead projectors to project a computer generated color image onto a projection screen for viewing by a large audience.
One typical configuration of color LCD displays comprises a series stacked assembly of three electronically switchable elements, which are here termed "color filter components." Light is projected through the display from a backlight panel, overhead projector or other light source. Each of the filter components operates to selectively attenuate the incident light in one of three spectra corresponding to the three primary colors: red, green and blue. The filter components operate collectively to attenuate selected combinations of the three primary colors while the unattenuated colors of light pass through. Attenuating different combinations of the primary colors produces light of various colors. For example, attenuating red causes the display to transmit cyan light (the combination of green and blue light). Attenuating red and green causes the display to transmit blue light. Color LCD displays of this type are described by Silverstein et al. in U.S. Pat. No. 5,032,007, Conner et al. in U.S. Pat. No. 5,050,965, and Mathewson in U.S. Pat. No. 5,122,887.
In the above cited patents, each switchable color filter component comprises an LCD panel interposed between first and second dichroic linear polarizers. The LCD panel and polarizers cooperate to selectively attenuate a predetermined primary color (spectral region of light) responsive to an applied signal. In general, the first "entrance" polarizer imparts a particular linear polarization orientation (e.g. zero degrees) to entering light in the predetermined spectral region. Entering light in the spectral region having an orthogonal linear polarization (e.g. ninety degrees) is absorbed by the entrance polarizer. The polarized light then passes through the LCD panel, which varies the polarization state in the predetermined spectral region. The second "exit" polarizer only transmits the light having a polarization parallel to its own polarization axis. Light of an orthogonal polarization is absorbed. Thus, by applying an appropriate signal, the predetermined primary color can be either passed or attenuated by the LCD filter component.
Although described in the foregoing paragraph as a single optic element, the LCD panel usually defines a plurality of separate, independently operable pixels. Electrical signals can be individually applied to each pixel to control the polarity of light passing therethrough to effect selective attenuation of the light in that filter component's predetermined spectral region. When filter components are placed in a series stacked assembly to form a color LCD display, the pixels of the separate filter components are aligned. In this manner, each pixel of a color image is produced by light which passes through an aligned set of LCD panel pixels, one in each filter component's LCD panel. The LCD panel pixels in the aligned set operate in series to subtract red, green, and blue as desired from entering light to produce a net color for each pixel in the image.
Ideally, each of the filter components should operate as a perfect notch filter (i.e. one which completely attenuates any incident light in a predetermined stopband spectral region and passes unattenuated any incident light in a passband region outside the stopband region). Color LCD displays with switchable color filter components of the above described configuration perform adequately to produce color images. However, due to limitations inherent in presently available materials, such filter components fail to provide performance even roughly approximating notch filters. As a result, the color LCD displays constructed with these filter components have suffered certain drawbacks, including diminished brightness and lower contrast. Diminished brightness results from the partial attenuation of light outside a filter component's stopband regions, i.e. in its passband regions. Lower contrast (the ratio of the luminance of the brightest displayable color, white, to the darkest, black) results from the failure of the filter components to completely attenuate light in their stopband regions and from having overly broad transition band regions (between the stopband and passband regions). The filter components insufficiently attenuating stopbands and overly broad transition bands result in leakage of light when all three filter components of a color LCD display are operated collectively to form black by attenuating all three primary colors. This results in a black which is too bright and, consequently, lowers display contrast.
To increase contrast, the three color stopbands of the filter components in some prior displays have been partially overlapped. The overlapped stopbands increase the darkness of black by preventing leakage of light between the stopbands. This overlap, however, also diminishes independent control of the three colors, particularly when the filter component's transition bands (stopband skirt) are wide. Additionally, overlapping the filter component's stopbands can result in poor color brightness since each overlapping stopband is too wide to allow good transmission of light in the spectral regions controlled by the other filter components.
For example, the stopband of a magenta filter component (which attenuates green light) can be widened so that it overlaps the blue and red stopbands of the other filter components in a display. This overlapping prevents leakage of light when displaying black, since transition regions between the stopbands are reduced or eliminated by the overlap. However, when red is to be displayed, the wider, overlapping green stopband of the magenta filter component also attenuates some of the red light. Accordingly, the brightness of red produced by the display is lessened.
A further drawback to the above described filter components is their poor thermal performance. The dichroic linear polarizers included in the filter components are generally formed from a plastic sheet containing dichroic dyestuff which is stretched in a particular direction to align the dyestuff molecules along a particular axis. The radiant energy of light whose polarization is oriented along the same axis is absorbed by the dyestuff, whereas light polarized perpendicularly is passed. (By using a colored dyestuff, only polarized light of a particular color is absorbed.) The absorbed light energy is thereby converted into heat energy. Consequently, when used in a color LCD display for projection, the dichroic linear polarizers cause the display to heat up rapidly. The display temperature can eventually increase to the point that display performance deteriorates or is impaired.
Various other configurations of electronically switchable color filter components are known. A filter component comprising a twisted nematic LCD panel sandwiched between same-handed cholesteric liquid crystal polarizers is described in Maurer et al., "Polarizing Color Filters Made From Cholesteric LC Silicones," SID 90 Digest, 1990, pp. 110-113 and in Schadt et al., "Novel Polarized Liquid-Crystal Color Projection and New TN-LCD Operating Modes," SID 90 Digest, 1990, pp. 324-326. In contrast to dichroic linear polarizers, which absorb light with a particular linear polarization orientation, cholesteric liquid crystal polarizers reflect a particular handedness of circularly polarized light in a characteristic region of the visible light spectrum. Since the light is reflected rather than absorbed by such polarizers, little or none of the incident light is converted to heat energy.
Another problem of prior stacked color LCD displays is that of parallax. As discussed above, pixel colors of the image produced by the displays are the result of light passing through an aligned set of filter pixels. Parallax is a visual effect resulting from viewing an image formed by light passing through misaligned filter pixels. Prior stacked color LCD displays have been constructed with their active LCD layer supported on relatively thick glass substrates, resulting in an effective thickness of the stacked filter components that is substantially greater than the width of the pixels. Because of the thickness of the stacked filter components, prior subtractive color LCD displays can only be viewed directly from within a narrow angle to avoid parallax. In such prior stacked LCD displays, it has been necessary to employ lenses which collimate light entering the display to avoid parallax effects. As a result of the parallax problem, such displays are unacceptable for direct viewing and have generally been limited to projection systems.
A further problem associated with stacked LCD displays involves depth of focus. Since the separate filter components are physically offset (i.e. misregistered), the controlled colors of light are focused at differing depths in the projected image. This difference in focus of different colored light in a projected image can be detected by a viewer. A solution to this problem involving the use of a stacked assembly of dichroic mirrors, one for each controlled primary color (e.g. red, green and blue) is described by Mathewson in U.S. Pat. No. 5,184,234. Light projected through the display is reflected from the stacked dichroic mirror assembly onto a viewing screen such that the offset reflections of each primary color by the stacked mirror assembly corrects the perceived depth of focus problem in the viewed image. However, with prior stacked LCD displays which employ filter components with overlapping stopbands to improve contrast as described above, the color of light controlled by one filter component in the display may be influenced by other filter components. When such a display's depth of focus is compensated by the stacked dichroic mirrors, this cross interference of other layers with the controlled color results in a ghosting effect or apparent shadow to objects in the image.
The present invention provides optical notch filters of novel configuration which apply circular polarization techniques and materials to produce more effective filter shapes. The resulting optical notch filters have filter shapes with better transmitting passband regions, better attenuating stopband regions, and narrower transition regions than the prior filter components described above. In general, the optical notch filters include a first element which selectively imparts a first or second circular polarization to transmitted light in a predetermined spectral region ("controlled band"), and a second element which likewise passes light of only one circular polarization within the controlled band.
In accordance with one aspect of the invention, the optical notch filter comprises a variable zero/half wave retarder element interposed between circular polarizers. Incident light in the controlled band is polarized to a first circular polarization by the first of the circular polarizers. The variable retarder is switchably responsive to an applied signal between zero retardation and half wave retardation so as to selectively vary the circular polarization of light exiting the retarder. The second circular polarizer transmits only one circular polarization of light in the controlled band while substantially completely attenuating the other. The optical notch filter thus provides selective filtering of light in the controlled band.
In accordance with another aspect of the invention, the variable zero/half wave retarder of the optical notch filter comprises an LCD that exhibits an electronically controllable birefringent effect, such as a super twisted nematic LCD (STN-LCD) element. The STN-LCD's electronically controllable birefringent effect is used to selectively vary the circularly polarized entering light between right and left handed circular polarizations. Cholesteric liquid crystal (CLC) polarizers are used as the first and second circular polarizers. The STN-LCD element and the CLC polarizers are tuneable to a particular spectral region and provide an optical notch filter with a particularly selective filter shape.
In accordance with a further aspect of the invention, a color LCD display is formed from a stacked assembly of the optical notch filters, each of which operates to selectively attenuate light in a respective portion of the visible spectrum. In preferred embodiments of the invention, the optic notch filter components of the stacked assembly are constructed with the STN-LCD and CLC polarizer layers supported on plastic substrates. With such a construction, the stacked assembly can be formed sufficiently thin to avoid parallax at acceptably large direct viewing angles without the need for collimating lenses. The resulting color display is lightweight and brighter, being particularly useful in low weight, low power consumption applications, such as for portable computer displays.
Another advantage of the stacked assembly formed according to this aspect of the invention is that with the improved filter shape of the filter components, there is much less cross-talk or interference with a filter component's controlled color by the other filter components in the stacked assembly. Consequently, when the depth of focus of a display formed with the stacked assembly is compensated by stacked dichroic mirrors, there is little or no perceived ghosting effect.
In accordance with another aspect of the invention, the display formed from the stacked assembly of notch filter components is configured to doubly analyze each color band, thereby improving display contrast.
In accordance with yet another aspect of the invention, a circular polarizer of the optic notch filter component is operated in conjunction with a linear polarizer and retarder element to more completely attenuate light in the filter component's spectral region.
In accordance with yet another aspect of the invention, the optical notch filter component is configured to operate uniformly across a broad spectral band in at least one of its birefringent states. This achromatic operation produces a more uniform attenuation or transmission of light in the filter component's spectral region. In one embodiment, the optical notch filter component comprises oppositely handed circular polarizers so that the achromatic operation is produced when attenuating light in the filter component's spectral region. A stacked assembly of filter components according to this embodiment produce a more uniform black.
In accordance with still another aspect of the invention, a selectively circularly polarizing element is provided which comprises a linear polarizer, a switchably birefringent layer (such as a STN-LCD layer), and a second birefringent layer. Incident light projected through the element is first linearly polarized to a predetermined orientation by the linear polarizer. When the light then passes through the birefringent LCD layer, elliptically polarized light results. (STN-LCD layers, in particular, tend to produce such a result.) The elliptically polarized light can then be adjusted to a circular polarization of light with the second birefringent layer. The birefringent LCD layer can be selectively operated to produce two different elliptical polarizations of light which, when passed through the second birefringent layer, result in right or left handed circular polarizations. The selectively circularly polarizing element thus formed can be used in conjunction with a circular polarizer to operate as a notch filter.
Additional features and advantages of the invention will be made apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the accompanying drawings.